Traditional and manipulated LB deposition methods have been reported to yield high-quality and large 2D rGO or GO thin films. The traditional method involves vertical dipping of the substrate through a water subphase during the close-packing arrangement of rGO sheets 
. For efficient material adhesion and for effective draining of water, a vertical dipping process could be carried out at a very low speed to increase adhesion time, as rGO will not attach well to the substrate at high dipping speeds 
. The resulting 2D rGO thin film is then air-dried at room temperature or kept in an oven to enhance the drying process 
The traditional method can be manipulated in several ways to deposit large-area 2D GO films. Xu and co-workers demonstrated a controlled edge-to-edge assembly of a floating 2D GO monolayer with the ability to yield large-area films 
. The method includes an improved spreading of solvent using ethanol/1, 2-dichloroethane with a volume ratio of 1:13 instead of the traditional methanol/water (ratio 5:1). As a result, the transfer efficiency increased by five-fold and a large accumulation of GO monolayer was achieved.
The mechanism of controlled edge-to-edge assembly contributes to barrier-free densification, the aggregation mechanism and spreading-induced film. The barrier-free densification of the LB film is accomplished by using repetitive dripping deposition of 0.025 mg/mL GO dispersion at the air–water interface. This method proves to be nearly 100% efficient in transferring a GO monolayer onto a large-area substrate at the air–water interface and is attributed to the immersion forces that occur while the spreading solvent evaporates. The aggregation mechanism is carried out by depositing GO dispersion at a low pH onto a highly ionic subphase, which results in the shielding of repulsive electrostatic forces between individual GO sheets. Spreading-induced film densification is achieved by spreading solvent at a suitable distance-dependent force from floating GO films. Compression of barriers will then effectively assist to form a densely tiled film. The distance-dependent force is transmitted throughout the LB trough dissimilarly depending on whether the interaction between the sheets is governed by repulsive or attractive forces.
Using this mechanism, a continuous deposition of 2D GO film onto a large-area substrate for coating and patterning using roll-to-roll deposition was achieved 
. In another report, the quality of the 2D films was significantly improved by manipulating the monolayer transfer process by adjusting the solid substrate at a slight angle during the dipping 
. This observed improvement was due to the beneficiation of shear modulus causing GO to behave like a 2D substrate on the water subphase as a result of the strong interaction of each individual GO sheet at the air–water interface. Furthermore, the implementation of a shallow-angled substrate during the dipping process projects a minimum tensile stress during the insertion and extraction of the substrate (Figure 2
). Poor GO adhesion onto hydrophobic surfaces resulting in low-quality film formation or coating when using a conventional vertical dipping method can be overcome by adjusting the dipping angle of the substrate to 30°, which creates a desirable meniscus radius of curvature.
A comparison of conventional (a
) 90° and (b
) 30° dipping mechanisms of upstroke and downstroke of different hydrophobic and hydrophilic substrates. The SEM images show the comparison of the GO film formations in the upstroke (c
) hydrophilic and (d
) hydrophobic substrates of Au at both 90° and 30°. (e
) SEM film formations as a result of downstroke using a hydrophobic Au substrate. Reprinted with permission from the Journal of the American Chemical Society, copyright 2015, American Chemical Society 
Three-dimensional graphene-based materials, meanwhile, are of particular interest due to their intrinsic properties similar to 2D graphene sheets while they also have the added benefits of increased surface areas combining meso-, macro- and micropores. The meso- and microporosity display a highly specific area and macroporosity significantly improves the catalytic performance at the surface region 
. These 3D porous structures coupled with the intrinsic properties of graphene impart high specific surface areas, durable mechanical strength and fast mass and electron transport kinetics. In line with this, several non-LB techniques have been developed to fabricate 3D structures. These include step-by-step assembly methods using graphene, GO or rGO sheets and direct synthesis from a carbon source (methane, ethanol or sugar) 
In a recent report, an unconventional approach was employed using the LB method for preparing 3D rGO films 
. While dipping is conventionally carried out at the solid-state target pressure for preparing 2D LB film (Figure 3
), Jaafar and co-workers carried out the deposition beyond the target pressure up to the breaking point. Beyond the breaking point, collapse or breakdown in the arrangement of the monolayer results in the formation of multilayers, providing an overall 3D morphology. Wrinkling effects are predominant while electrostatic repulsion between the 2D layers increases the average thickness 
. Moreover, this may also increase the average roughness when the sheets crumble and wrinkle unevenly at different areas. Repeated transfer or layer-by-layer deposition further improves the 3D structures.
Flowchart showing the preparation of rGO LB film. To distribute the rGO flakes evenly, methanol vapor from a methanol-soaked tissue was applied to gently force the clouded regions apart to significantly improve the film uniformity. Reprinted with permission from ACS Publications, copyright 2015, American Chemical Society 
Annealing at 60 °C between each consecutive deposition of layers helps to create higher surface roughness and improves the porosity of the 3D structures. It was reported that at the sixth deposition layer of rGO, complex interconnected micrometer-scale porous structures were achieved which enabled enhanced integration and encapsulation of microalgae within BPV FCs for increased power output 
. At higher deposition cycles, the roughness and pore size were increased, thus adjustment of the deposition cycles could be a useful way of achieving various roughness and pore sizes of 3D structures. These tunable properties may play important roles in cellular interactions by influencing cell behavior in biological microenvironments, such as in the rGO biofilms 
. Compared to 2D graphene-based films, 3D structures provide excellent cellular communication, transportation of oxygen and nutrients, removal of waste and improved cellular metabolism. Furthermore, high porosity allows significant improvement in the adhesion of biomaterials and, in the case of microalgae, allows enhanced growth. As such, the LB technique provides a practical solution for controlling and optimizing surface roughness and pore size with respect to its intended applications.